Dividing cells must co-ordinate their rate of division with their rate of cell growth and protein synthesis. In S. cerevisiae, the G1 cyclin C1n3 is central to this co-ordination. Cells use the amount and rate of synthesis of C1n3 to measure readiness for cell division. When cells achieve "critical size," C1n3 activates the transcription factors SBF and MBF, which in turn induce the transcription of over 200 genes. These 200 genes are directly responsible for cell cycle progress into S-phase. It is not understood how small changes in C1n3 abundance or concentration at "critical size" are translated into activation of SBFand MBF-dependent genes, and this is the issue addressed in this proposal. Experiments in Aim 1 will discover the molecular connection between Cln3 and the transcription factors SBF and MBF. Experiments in Aim 2 will characterize the changes that occur at SBF-dependent promoters as a consequence of C1n3 activity. Experiments in Aim 3 will characterize several proteins that interact genetically or physically with Cln3, and which may help environmental and physiological events signal to C1n3, or may help C1n3 signal to its target transcription factors.